The iPad's screen is polarized in such a way that the screen looks black when you are wearing polarized sunglasses and hold it vertically.
The iPhone is polarized so that the screen goes black when the phone is at 45 degrees; there is some loss of brightness when it's held horizontally, but it's still usable.
I don't face this problem, but just found it when scrolling down the forum.
It's just funny to have not being to see the screen when wearing certain sunglasses.
However, it's just a piece of cake to take down your sunglasses and focus use on your iPad.
(Why to wear sunglasses when using iPad?, duh..)